(a) Field of the Invention
The present invention relates to a high-speed high-temperature nematic liquid crystal composition and a liquid crystal display comprising the same, and particularly to a high-speed high-temperature nematic liquid crystal composition comprising a nematic crystal liquid compound that has a high phase liquid crystal transition temperature, large birefringence and elasticity coefficients, and a broad operating nematic phase temperature range, and thus can realize a high response speed and is effective for various devices requiring liquid crystal such as LCDs, etc., and a liquid crystal cell comprising the same and a liquid crystal display using the same.
(b) Description of the Related Art
Generally, a liquid crystal display is a device in which liquid crystal material is injected between an upper substrate on which opposite electrodes and color filters, etc. are formed, and a lower substrate on which a thin film transistor and pixel electrodes, etc. are formed, and wherein different electric potentials are applied to the pixel electrodes and the opposite electrodes to form an electric field to change an arrangement of liquid crystal molecules, thereby controlling transmissivity of light to embody images.
Liquid crystal displays are divided into TN (Twist Nematic), STN (Super Twist Nematic), and ferroelectric liquid crystal displays of the passive matrix method, and TFT (Thin Film Transistor), MIM (Metal Insulator Metal), and diode liquid crystal displays of the active matrix method, according to operation type.
For an active matrix method, high voltage maintenance with a low leak current is important because a liquid crystal display is operated with switching devices such as TFTs or MIMs for each pixel. In addition, a liquid crystal display tends to require a high response speed due to an increase in display information amount and for embodiment of moving pictures.
The thin film transistor substrate is used as a circuit substrate for independently operating each pixel in a liquid crystal display or an organic electroluminescence (EL) display, etc. On the thin film transistor substrate, scanning signal wiring or gate wiring for transmitting scanning signals, and picture signal lines or data wiring for transmitting picture signals are formed, and it comprises a thin film transistor connected with the gate wiring and data wiring, pixel electrodes connected with the thin film transistor, a gate insulating film that covers the gate wiring to insulate it, and a protection film that covers the thin film transistor and the data wiring to insulate them. The thin film transistor comprises a semiconductor layer that forms a gate electrode and a channel, which are parts of gate wiring; a source electrode and drain electrode, which are parts of data wiring; and a gate insulating film and a protection film, etc. The thin film transistor is a switching device for transmitting or blocking picture signals transmitted through data wiring according to scanning signals transmitted through gate wiring.
In addition; a nematic liquid crystal composition is used as an important material for a liquid crystal display (LCD) used for electronic computers, electronic notebooks, personal computers, etc. And recently, it has widely spread as a display for high speed information treatment for word processors, personal computers, etc.
However, because nematic liquid crystal compositions known so far have a low response speed, it is difficult to adequately embody moving pictures with them.
In order to solve these problems, the following requirements should be satisfied. First, in order to improve a response speed, the viscosity of a liquid crystal material should decrease to a range of 20 to 25 mm2/s. Second, in order to lower an operating voltage, a dielectric anisotropy (Δ∈) thereof should increase to a range of 10 to 15 (35° C., 1 kHz). Third, a liquid crystal material should have a nematic phase over a broad temperature range, and preferable the nematic phase temperature range is −30 to 80° C. Forth, birefringence (Δn) should be 0.20 (25° C.) or more.
In addition, although the LCD has physical advantages of lightness and smallness, brightness, which is one of various factors determining picture quality, is weak compared to a CRT (Cathode Ray Tube). Additionally, as the LCD-TV market is attracting attention as a large market for monitors, and new LCDs are being produced and disseminated, requirements for high brightness and high response speed become very important.
In order to achieve high brightness in an LCD, the phase transition temperature of liquid crystal should be maintained high due to a tube current in the back light. Also, for a high response speed, the rotation viscosity of the material should decrease or the refractive index of liquid crystal should increase.
Most LCD products marketed to date use nematic liquid crystal such as TN, IPS, and VA modes, and the phase transition temperature thereof is about 70 to 80° C. and response speed is about 20 to 30 ms. However, since the phase transition temperature and response speed are still unsatisfactory, improvements in response speed and increases in the phase transition temperature are needed in order to realize TV applications and moving pictures.
In addition, it is well known that a liquid crystal material having high birefringence (Δn) and elasticity coefficients is required in order to improve electro-optical characteristics of the TN-LCD, STN-LCD, and TFT-LCD. However, although liquid crystal compounds so far known can improve electro-optical characteristics of liquid crystal material, problems still remain in chemical safety of liquid crystal material and the operating temperature range of a liquid crystal display because if the liquid crystal compounds are used, birefringence of the mixed liquid crystal increases but a smectic phase easily appears or the operating nematic phase temperature range is narrow.